Soil water monitoring

INTRODUCTION ...1MEASURES OF SOIL WATER STATUS ...2 Gravimetric, volumetric and potential measures ...2 Water depth...2 Variability ...3 TECHNOLOGIES FOR MEASURING SOIL WATER STATUS ...4

SOIL WATER MONITORING

P Charlesworth,CSIRO Land and Water

SEPTEMBER2000

IRRIGATION

INSIGHTS

Biblio

The author acknowledges the following:

Liz Humphreys for editorial help

The Victorian Branch of the Australian Soil Science Society Institute for tions to the technical papers in the appendixes

contribu-Paul Hutchinson (CSIRO Land and Water), Robert Hoogers (NSW Agriculture)and David Williams (NSW Agriculture) for contributing to written sections

Richard Wells, Don Murray (Coleambally Irrigation), Nick Austin (NSWAgriculture), Iva Quarisa (NSW Agriculture) and Richard Stirzaker (CSIRO Landand Water), for providing constructive review comments

PUBLISHED BY

Land and Water AustraliaGPO Box 2182

Canberra ACT 2601Phone: 02 6257 3379Fax: 02 6257 3420Email: <public@iwa.gov.au>

DISCLAIMER

The information contained in this publication has been published by Land andWater Australia to assist public knowledge and discussion and help improve the sus-tainable management of land, water and vegetation Where technical informationhas been provided by or contributed by authors external to the corporation, readersshould contact the author(s) to make their own enquiries before making use of thatinformation

INTRODUCTION 1

MEASURES OF SOIL WATER STATUS 2

Gravimetric, volumetric and potential measures 2

Water depth 2

Variability 3

TECHNOLOGIES FOR MEASURING SOIL WATER STATUS 4

Porous media 4

Tensiometers 4

Resistance blocks 5

Combination devices 5

Wetting-front detectors 5

Soil dielectric 6

Time domain reflectometry 6

Frequency domain reflectometry 6

Neutron moderation 7

Heat dissipation 7

SELECTING A PRODUCT 8

Accuracy of equipment 8

SUMMARY TABLE OF PRODUCT FEATURES 9

PRODUCT DESCRIPTIONS 18

POROUS MEDIA 18

Tensiometers measured by handheld transducer 18

Gauge-type tensiometers 20

Gypsum blocks 21

Moisture-activated irrigation system 22

Granular matrix sensor 23

SOIL MATRIC POTENTIAL THERMAL HEAT SENSOR (CAMPBELL 229) 24

Equitensiometer 25

FREQUENCY DOMAIN REFLECTOMETRY (CAPACITANCE) 26

EnviroSCAN® 26

Diviner 2000® 28

C-Probe® 29

Gopher® 30

Buddy® 32

Aquaterr® 33

ThetaProbe® 34

Netafim soil moisture data collector 35

TIME DOMAIN REFLECTOMETRY (TDR) 37

Tektronix 1502 TDR cable tester 37

TRASE TDR 38

Campbell Scientific TDR100 39

Water content reflectometer (Campbell 615) 40

Aquaflex® 41

Gro-Point® 42

NEUTRON MODERATION 43 Contents

CASE STUDIES 49

Water use by furrow-irrigated onions on a clay soil 49

Rooting depth of irrigated rockmelons on clay soils 51

Swelling soils: problems with access-tube installations 54

Pitfalls of soil water monitoring: from dam building to damned drippers 56

When will I irrigate? Technology to aid irrigation-scheduling decisions on dairy farms 60

Field use of TDR and tensiometers 63

Tensiometer scheduling performance 63

The value of continuous data for implementing effective and efficient irrigation management 64

CONTACT LIST 69

WEB RESOURCES AND FURTHER READING 70

Web resources 70

References and further reading 70

APPENDIX 1 72

PRICELIST FOR ADDIT C-PROBE SOIL MOISTURE SYSTEM .72

APPENDIX 2 74

TIME-DOMAIN REFLECTOMETRY: AN INTRODUCTION 74

APPENDIX 3 82

FREQUENCY DOMAIN REFLECTOMETRY 82

APPENDIX 4 84

NEUTRON MODERATION METHOD (NMM) 84

APPENDIX 5 92

A VALUE SELECTION METHOD FOR CHOOSING BETWEEN ALTERNATIVE SOIL MOISTURE SENSORS 92

APPENDIX 6 95

ANNUAL CROP SOIL-MOISTURE-MONITORING COST COMPARISON 95 Contents

1 Introduction

Irrigators are under increasing pressure to manage water more prudently and moreefficiently This pressure is driven by product quality requirements, economic factors,demands on labour and the desire to minimise the resource degradation and yield lossthat can result from inefficient irrigation The need for farmers to irrigate more effi-ciently has led to an explosion in the range of equipment available for measuring soilwater status

The key to efficient on-farm irrigation water management is a good knowledge ofboth the amount of water in the soil profile that is available to the crop and theamount of water the crop needs Measuring and monitoring soil water status should

be essential parts of an integrated management program that will help you avoid theeconomic losses and effects that under irrigation and over irrigation can have on cropyield and quality They will also help you to avoid the environmentally costly effects

of overirrigation: wasted water and energy, leaching of nutrients or agricultural icals into groundwater supplies, and degradation of surface waters with contaminatedirrigation water runoff

chem-No existing resource offers a comprehensive, one-stop guide to all the available soilwater sensing and monitoring equipment Irrigation extension staff, consultants,equipment sales people and irrigation managers face a huge task in finding out aboutthe range of technology available and becoming familiar with the features, advantagesand limitations of each system By having the information at their fingertips they canmore easily match the equipment to the required task and budget

This Irrigation Insights information package brings together information on current

equipment and techniques for measuring and monitoring soil water status, extending

to their use as controllers in automatic irrigation systems We have limited the ment described here to those products with agents and backup within Australia Thehub of the publication is a collection of tables summarising the main product features.This enables you to compare product features As well as technical data, there is alsocommercial information on suppliers, contact details, availability and price (accurate

equip-at August, 2000) Case studies from personal experience and from the literequip-ature vide further insight into the advantages and limitations of each device in relation toits potential applications

pro-1 Introduction

2 Measures of soil water status

GRAVIMETRIC, VOLUMETRIC AND POTENTIAL MEASURES

There are three common ways to describe the wetness of soil: gravimetric soil water content (SWC), volumetric SWC, and soil water potential Which description is used

depends partly on how the information will be used You can use all three methods forthe same purpose, i.e to work out whether you need to irrigate

Gravimetric SWC refers to how much water is in the soil on a weight basis, for

example, 0.3 g water per 1 g of dry soil This is the easiest way to measure SWC Allyou do is take a small soil sample, weigh it, dry it in an oven for a day, and then weigh

it again The weight difference is the water extracted from the sample

One problem with gravimetric measurement is that the densities of different soilsvary so a unit weight of soil may occupy a different volume To allow you to comparethe water contents of different soils and to calculate how much water to add to the soil

to satisfy a plant’s requirement, you need to do a volumetric measurement.

Volumetric SWC is the most popular method of reporting the moisture status of

soil It is calculated by multiplying the gravimetric SWC by the soil bulk density, and

it uses units of cubic centimetres (or millilitres) of water per cubic centimetre of soil.The bulk density is the mass of soil solids per unit volume The bulk density is alsoused to calculate how much water a soil can hold

Volumetric measurements are convenient for measuring how full the soil is, butthey give no indication of how difficult the water is to remove As the soil becomesdrier, the water is held more tightly and more energy is needed to extract it The soil

water potential is a measure of this tension and is expressed in kilopascals (kPa) Potential is also referred to as soil water suction This is the term used in the package.

Irrigation can be managed to maintain soil water suction within the correct range sothat the crop is not stressed However, trial and error are needed to determine the vol-ume of water to be added

As an introduction to these measurements, Table 1 shows the average values for arange of soil textures

TABLE 1

content and soil water suction values (kPa).

WATER DEPTH

Most irrigators refer to water applied to a crop in volumetric terms, for example, inmegalitres per hectare (ML/ha) or Dethridge wheel revs (rpm)

Application rates can also be expressed in terms of depth, for example, millimetres

A water depth is merely a volume averaged over a land area For instance, 1 ML

2 Measures of soil

water status

Gravimetric, volumetric and

potential measures

Bulk density = Bulk density = Bulk density =

applied over 1 ha is equivalent to 100 mm This allows comparison with factors such

as rainfall and crop water use or evapotranspiration For example, as an aid to tors in the Murrumbidgee Irrigation Area, NSW, the potential crop evapotranspira-tion (in mm) is included in the nightly weather report Using a simple calculation, thisfigure can be converted to a volume to be applied to the crop

irriga-VARIABILITY

Agriculturalists are very aware of the variability that exists in their systems In fact,they put a lot of effort into trying to even out this variability and get a uniform prod-uct Subtle and sharp changes in soil type are evident both across the paddock anddown the profile Variations in crop growth can point to soil changes, past paddockuse, disease, or irrigation application problems such as blocked drippers Even veryclose to a plant there will be variations in where the plant extracts its water from Timebrings in another level of variability, with differences throughout the day and season

in where and how much water is being extracted from the soil Consider, for example,

a row of drip-irrigated grapevines Not only is there soil variation to contend with, butalso variation in the amount of water applied between the drip emitters: from very wet

at the emitter to drier in between Impose on this a row of plants that alter where theirrigation water spreads, and you can see that the system is quite complex

All the available soil-water-measuring instruments can tell us the soil water status

at a particular point in a paddock If you have a number of sensors, then you can place

an array throughout the profile to give more information However, because there arepractical limitations in the wiring or in the time taken to read them, you will gener-ally have to place them close together Depending on the soil-water-monitoring systemthere may be only a single reading every few hectares This reading has to average outall the variability present in the whole area That is, you are assuming that the instru-ment is placed in the average soil type, next to the average plant, at the depth of aver-age water uptake and in the zone of average water application You then design an irri-gation schedule to satisfy the plant and soil in this position Even if there are enoughsensors present to show the variation in the field, how do you respond to the varia-tions? Do you water to satisfy the driest part of the field, ensuring no plant is under-watered? Or do you water to the wettest instruments, thus using water very efficient-

ly but at the risk of decreased yield?

All these issues must be taken into account when you are designing a monitoring system We strongly recommend that you talk to someone experienced inthese matters, such as a consultant or irrigation officer, before you go ahead

soil-water-Variability

Soil type, crop density, disease,

application uniformity, time of

season - all cause variation the

irrigator must deal with.

INSTALLATION IS

CRITICAL!

For best results, it is important

that the instrument is placed in

the average soil type, next to the

average plant, at the depth of

average water uptake and in the

zone of average water

application.

3 Technologies for measuring soil water status

In this package we use the following definition of a soil water sensor:

A soil water sensor is an instrument which, when placed in a soil for a period of time, vides information related to the soil water status of that soil (Cape 1997).

pro-Gravimetry (in this case drying soil samples and then weighing them) is the only

direct way to determine how much water is in the soil All other techniques rely on indirect methods that measure other properties of the soil that vary with water content.

The 24 products listed in this section all exploit one of the following indirect urement systems for measuring the soil moisture status:

meas-a) suction

• porous media instruments

• wetting-front detectorsb) volumetric water content

• soil dielectric: time domain reflectometry, frequency domain reflectometry (FDR or capacitance)

Porous media instruments measure soil water potential and take three forms:

> tensiometers

> resistance blocks

> combination volumetric SWC – porous material devices

The range of measurements that can be achieved with these types of devices isshown in Figure 3.1

Tensiometers

A tensiometer is an instrument that directly measures soil moisture potential It sists of a porous ceramic tip, a sealed water-filled plastic tube and a vacuum gauge(Goodwin 1995) The porous cup is buried in the soil and allows water to move freelybetween the water-filled tensiometer and the soil As the soil around the cup dries, thepotential increases, and water moves out of the tensiometer until the potential withinthe tensiometer is the same as that of the soil water

con-Since the tensiometer is an airtight device (Figure 7.1), as water moves out from theporous cup a negative pressure (a vacuum or suction) equivalent to the soil potential

is created in the tensiometer If the soil around the tensiometer becomes wetter (forexample, from rain or irrigation) the soil potential decreases, and soil water flowsthrough the porous walls of the cup into the tensiometer, decreasing the suction The soil suction reading relates directly to the amount of energy a plant must use

to remove water from the soil, and hence is a more meaningful measure of plant stressthan the soil water content The suction is measured with a vacuum gauge or pressuretransducer The transducer can either be a handheld device (used to read many ten-

3 Technologies for

measuring soil water

status

Porous media

siometers manually) or be permanently installed in the tensiometer and connected to

a logger The portable device has a hollow needle that is inserted through a rubberbung or septum to measure the vacuum

Tensiometers cannot be used to measure soil water suction greater than 75 kPa.Suctions above this cause the vacuum in the tensiometer to break down, as air entersthe ceramic tip They are fine for most annual vegetable crops, orchards, nuts and pas-tures, but they are not adequate for the controlled stressing of plants such asgrapevines, where suctions as high as 200 kPa are recommended to produce good winequality

Resistance blocks

Resistance blocks consist of two electrodes embedded in a block of porous materialthat is buried in the soil As with tensiometers, water is drawn into the block from awet soil and out of the block from a dry soil The electrical resistance of the block isproportional to its water content, which is related to the soil water potential of the sur-rounding soil

Combination devices

Several of the soil water suction sensors consist of volumetric SWC sensors embedded

in porous materials with known water-retention properties The water content of thematerial equilibrates with the suction of the surrounding soil and is measured by thesensor

FIGURE 3.1 Measurement ranges for several soil-water-tension monitoring instruments.

Key points:

> Tensiometers are suited to vegetable crops, orchards, nuts and pasture

> Gypsum blocks and granular matrix sensors are suited to Regulated Deficit

Figure 3.1

When the soil dries to below the set point the detector switches off Wetting-frontdetectors are cheap because they do not need to have continuous outputs that are cal-ibrated to the soil water content.

Wetting-front detectors provide useful information to irrigators in three main ways:

Warning signals

If a wetting-front detector is placed near the bottom of the root zone it canact as a warning signal that overirrigation is occurring Irrigation beyond thisdepth is wasted, because the crop cannot get access to this water Irrigatorscan use a wetting-front detector to reduce overirrigation, fertiliser loss andwaterlogging and, as a consequence, to increase crop yield

Regulation of amount of water irrigated

Wetting-front detectors can be used to regulate the amount of irrigation tothe crop’s water demand by placing the detector within the root zone andturning off the irrigation when the wetting front is detected This regulationoccurs because the wetting-front speed depends on how dry the soil is beforeirrigation If the soil is relatively dry, the wetting front moves slowly into thesoil This occurs because the soil absorbs much of the water and hence slowsthe progress of the wetting front Conversely, if the soil is already wet, thewetting front moves fast because the irrigation water finds little availablespace to occupy

Collection of soil-water samples

Wetting-front detectors can be designed to collect samples of soil water fromthe wetting front These samples contain solutes such as salt and nitrate.When analysed, these samples can provide useful information about manag-ing fertilisers and the leaching of salt from the root zone (Stirzaker andHutchinson 1999)

SOIL DIELECTRIC

The dielectric constant is a measure of the capacity of a non-conducting material totransmit electromagnetic waves or pulses The dielectric of dry soil is much lower thanthat of water, and small changes in the quantity of free water in the soil have largeeffects on the electromagnetic properties of the soil water media

Two approaches have been developed for measuring the dielectric constant of thesoil water media and, through calibration, the SWC: time domain reflectometry andfrequency domain reflectometry

Time domain reflectometry

The speed of an electromagnetic signal passing through a material varies with thedielectric of the material Time domain reflectometry (TDR) instruments (for exam-ple, TRASE/Tektronix) send a signal down steel probes (called wave guides) buried inthe soil The signal reaches the end of the probes and is reflected back to the TDR con-trol unit The time taken for the signal to return varies with the soil dielectric, which

is related to the water content of the soil surrounding the probe

TDR instruments give the most robust SWC data, with little need for recalibrationbetween different soil types However, they are extremely expensive and you may needadditional electronic equipment to run them

Frequency domain reflectometry

Frequency domain reflectometry (FDR) measures the soil dielectric by placing thesoil (in effect) between two electrical plates to form a capacitor Hence ‘capacitance’ isthe term commonly used to describe what these instruments measure When a voltage

Soil dielectric

is applied to the electric plates a frequency can be measured This frequency varieswith the soil dielectric.

FDR-type products have been the main area of expansion in the production of water-monitoring equipment

soil-All dielectric sensing products have a relatively small measurement sphere of about

10 cm radius, with 95% of the sphere of influence within 5 cm This makes them sitive to inconsistencies introduced during installation, such as air gaps beside accesstubes The Aquaflex®, developed in New Zealand, seeks to integrate such problemsover a large soil volume by making the single sensor very long (about 3 m)

sen-Within this product group a variety of installation methods are represented,including by access tube, portable sensors and buried sensors The Gopher®(see later)

is operated similarly to a neutron probe One sensor is lowered down an access tube tothe required depth It can then be moved to another location EnviroSCAN®also uses

an access tube, but it consists of an array of identical sensors placed permanently atset depths, offering the advantage of both time and depth series logging

To calibrate dielectric sensors, two-point (wet and dry) gravimetric sampling isused EnviroSCAN is provided with a ‘universal calibration’, but there is also a com-prehensive calibration procedure that can be used if you need greater accuracy

For more technical explanations of TDR and FDR see Appendix 2 and Appendix 3.

in soil water content

For a more technical explanation of the NMM see Appendix 4.

HEAT DISSIPATION

Heat capacity is the amount of heat energy needed to increase the temperature of aquantity of water by 1ºC Sensors in this category exploit the fact that water has a fargreater heat capacity than soil

Therefore, if a wet soil and a dry soil are subjected to an equivalent amount of heatenergy, the wet soil will experience a lower increase in temperature Sensors using thisprinciple consist of a heat source separated by a known distance (3 to 10 mm) from atemperature sensor They are buried at the depth of choice A burst of heat energy ofknown amount is emitted from the heat source As the heater is turned off, the tem-

ACCESS TUBE

INSTALLATION.

To ensure good data, tubes

must be installed to maximise

soil contact:

> A slightly undersized auger

should be used.

> A cutting edge should be

placed on the tube end.

> Tube should not be flexed

horizontally.

> In difficult soils a slurry

may be needed.

> Top of soil around tube

should be sealed to prevent

water intrusion.

Neutron moderation

Heat dissipation

4 Selecting a product

The products discussed in this publication are described by the following 19 attributes:

1 Reading range 8 Country of origin 15 Irrigation system suited to

2 Stated accuracy 9 Remote access 16 Best soil type

3 Measurement sphere 10 Link to other equipment 17 Application

4 Output reading 11 Interface to PC 18 Capital cost

5 Installation method 12 Affected by salinity 19 Annual operating cost

6 Logging capability 13 Expansion potential

7 Power source 14 Technical support

When you are selecting a product, choose the attributes most important to you fromthe above list and compare them for each product However, the dominant factor inthe selection process will not be physical/plant/soil based, but will invariably be thetrade-off between your initial capital investment and your ongoing labour cost Forexample, a tensiometer is relatively inexpensive but must be read daily and main-tained weekly A modern multi-depth logging system is relatively expensive, but datacan be sent straight to the office PC and viewed with little labour input A furtherintangible consideration is that it needs great discipline to maintain a regime of man-ual readings

To this end, an economic analysis of soil-moisture-monitoring equipment has been

included in Appendix 6 to demonstrate that the lifetime cost of a product should be

included in the selection process

Also included, as Appendix 5, is an example of a proforma for product selection that

incorporates both important attributes and economic aspects

As there is no universal, objective source of measured accuracy available, we haveused the manufacturers’ ‘stated accuracy’ in this package

4 Selecting

a product

Accuracy of equipment

5 Summary table of

product features

Table 5.1 describes the level of skill required for instrument operation

This objective score reflects the author’s opinion and is split into three levels :

4 Minimal skill - with a small amount of training a person is well able toaccomplish the task

44 Considerable - a large investment is required to become proficient The ence is that it may be more efficient to hire a consultant

infer-444 Specialist - either specialised equipment or a high degree of theoreticalknowledge is required

5 Summary table of product features

TABLE 5.1 Skill levels needed to operate different products

TABLE 5.2 Comparison of porous media technologies for measuring soil water tension.

TABLE 5.3 Comparison of frequency domain reflectometry (capacitance) technologies for measuring SWC.

maintenance @ $100 for first site,

inserted for manual readings

Neutron moisture meter

be recorded on-board for later download

Neutron moisture meter

6 Product descriptions

Porous mediaTENSIOMETERS MEASURED BY HANDHELD TRANSDUCER

Methodology

Meter-read tensiometers can be made or bought These tensiometers must be read with

a portable electronic vacuum gauge A needle connected to the gauge is inserted intothe rubber septum, and the reading is displayed on the meter (Figure 6.1) Transducerscan be added into the tensiometer through a T-piece so you can connect it to a logger.Directions for the construction of tensiometers are included in Goodwin (1995).The tensiometer must be airtight To test this, fill it with water, place it in the sunand read it every half-day A reading of 70 to 80 kPa should be reached before airenters the tensiometer and makes the reading to revert to zero

To install a tensiometer, auger a hole to the desired depth Then insert the ter and surround the tip with finely ground, tamped soil to ensure excellent contact.Fill the rest of the hole with a bentonite–soil mixture to ensure water doesn’t flowdown between the tensiometer and the soil

tensiome-Tensiometers are capable of reading to 80 kPa, but they become less accurate past

50 kPa, as the suction causes the water to de-air (Greenwood and MacLeod 1998)

Calibration

As tensiometers measure soil: water suction you do not need to calibrate them to thesoil type The transducer in the handheld meter is pre-calibrated to kPa No furthercalibration is required

Data handling

Meters are available with and without internal memory Manual readings can beeither recorded using graph paper or entered into a PC spreadsheet The computergauge (SoilSpec) comes with custom software to allow downloading, viewing and stor-age of readings

Maintenance

In a dry soil, water is drawn out of the tensiometer more quickly than in a wetter soil

If the level drops more than 2 cm from the top the readings become inaccurate Checkthe water level in the viewing tube at least weekly, and refill the tensiometer if neces-sary If it is located in a frost-prone area, you can add methylated spirits (50 mL/Lwater) to the tube to stop the water freezing The rubber septum perishes and degradesafter it has been pierced many times by the meter needle Cover it and replace it reg-ularly

Potential limitations

> Must have a meter to take measurements, as opposed to a gauge-type siometer

ten-> Manual data collection

> High maintenance requirement to maintain data quality

> Difficult to convert readings to soil water content Makes it harder to late the irrigation amount needed

calcu-6 Product descriptions

Porous media Tensiometers measured by

handheld transducer

SoilSpec, Terra Tech

> Measurement range limited to 0 to 80 kPa Becomes inaccurate after 50 kPa

> Removing the bung during refilling can lead to movement of the tensiometerand problems with loss of soil contact

Positive points

> Measures soil water tension – more meaningful from a plant-stress aspect

> Simple and cheap Easy to understand

> No cabling required (except where tensiometers are logged)

> One meter can be used to take readings at many locations/depths

> Better resolution in wetter soils than, for instance, gypsum blocks

> Data can be used without further calculations

> Not affected by salinity

FIGURE 6.1 Home-made tensiometer (left) Commercial tensiometer/

transducer products Terra Tech (top right), SoilSpec (bottom right).

Figure 6.1

Electronic Tensiometer Meter

Rubber Plug 13mm x 100mm

Maintenance is similar to that for meter-read tensiometers A vacuum pump is used

to remove trapped air from gauge-type tensiometers The Jetfill tensiometer has asmall reservoir that permits rapid refilling

Potential limitations

> Manual data collection

> More expensive than meter-read tensiometers

> High maintenance requirement to maintain data quality

> Difficult to convert to SWC Makes it harder to calculate the irrigationamount

> Measurement range limited to 0 to 80 kPa

Positive points

> External meter not required You can view the reading any time you pass thetensiometer

> Easier to maintain than meter-read tensiometers

> Measures soil water tension – more meaningful from a plant-stress aspect

> No cabling required (except where tensiometers are logged)

> Better resolution in wetter soils than, for instance, gypsum blocks

> Data can be used without further calculations

> Not affected by salinity

FIGURE 6.2 Gauge-type tensiometer.

Gauge-type

tensiometers

JetFill, Irrometer

Figure 6.2

GYPSUM BLOCKS

Methodology

Gypsum blocks consist of a pair of electrodes embedded in a block of plaster of Paris(Figure 6.3) Gypsum blocks are measured by a portable meter or remotely by a datalogger There are several different brands of gypsum blocks, each using different

dimensions These will all have different calibration characteristics and must use the

reader designed for them If you have a knowledge of electronics you can make yourown portable meter

To prevent polarisation of the block, use an alternating current circuit to measurethe resistance between the two electrodes One method is to apply an oscillating volt-age and measure, in series with a multimeter, the alternating current through the gyp-sum block Calibration curves that convert the current to soil water tension are avail-able for the various commercial gypsum blocks There are also several commercialmeters available

Before you install the gypsum blocks, soak them in water to remove any air ets Bury them at the required depth, as for tensiometers Place finely ground soilaround each block to ensure good contact, then backfill the hole with a soil–bentonitemix to stop preferential flow Mark the wires well and tie them to a stake or vine trel-lis

pock-Gypsum blocks buffer against the effects of salinity Determinations of soil watertension are not affected by salinity up to 6 dS/m (soil water solution), a figure higherthan the salt-stress level for most crops

Calibration

As the gypsum block measures soil water tension, you do not need to calibrate it toyour soil type The relationship between block resistance and soil water tension is verysensitive to block size, gypsum composition and electrode-separation distance.Therefore, it is better to use commercially available blocks to ensure uniformity

Data handling

Data is recorded by a handheld meter and manually recorded or stored for download

to a PC Soil water-tension data require no further calculations and can be comparedwith target figures for the specific crop and growth stage

Maintenance

Gypsum blocks are maintenance free, although as they dissolve their calibration erties change Depending on soil type, the amount of rainfall and irrigation and thetype of gypsum block, they should last from 1 to 8 years

prop-FIGURE 6.3 Gypsum blocks and readers/loggers.

Gypsum blocks

Figure 6.3

Potential limitations

> Insensitive to tension changes in wet soil (< 30 kPa)

> Must be read manually A logging system is available

> Measure soil water tension, which is good indication of when to irrigate, not how much.

> Blocks dissolve over time

> Do not work well in sandy soils, where the moisture drains more quickly thanthe time needed for the sensor to equilibrate

Positive points

> Simple and cheap

> Capable of reading to low (dry) tensions (about 1000 kPa) Therefore good fordrier soils and regulated-deficit irrigation

> Measure soil water tension – more meaningful from a plant-stress aspect

> Not affected by salinity up to 6dS/m (soil water solution)

MOISTURE-ACTIVATED IRRIGATION SYSTEM

Methodology

This prototype product consists of an electrical resistance sensor (twin probe) linkedthrough decision electronics to a solenoid, resulting in an ‘automatic’ irrigation sys-tem (Figure 6.4) The probe is measured similarly to a gypsum block but lacks the gyp-sum covering Hence we have included it with ‘porous media’

The probe is installed by being pushed into the soil

Probe resistance is calibrated to a ‘wet’, ‘OK’, or ‘dry’ water content As the

select-ed set point is passselect-ed, the controller switches the solenoid on or off to maintain an even

soil water content

Calibration

Calibration is done quantitatively The soil around the inserted probe is wet to thedesired depth and checked with a shovel The set point control is turned until thegreen ‘OK’ light turns on As the profile dries the ‘dry’ light turns on and the solenoid

is activated until the resistance decreases enough to light the ‘OK’ light again Manualirrigation is also catered for

Data handling

The system is self-contained and needs no external data-recording

FIGURE 6.4 Moisture-activated irrigation system.

Moisture-activated

irrigation system

Figure 6.4

at both different times and sites.

> Subjective choice of set point

> Limited to a single-probe decision for each solenoid

Positive points

> Cheap and simple

GRANULAR MATRIX SENSOR

Methodology

Granular matrix sensors use the same principle as the gypsum block Electrodes areembedded in a patented granular quartz material This is protected by a syntheticmembrane and then a stainless steel mesh (Figure 6.5) The material selected enablesthe sensor to measure wetter soil than a gypsum block (up to 10 kPa) The sensorincludes internally installed gypsum, which provides buffering against salinity effects.This type of sensor is installed via an augered hole Surround the sensor with fine soil,and backfill the hole with a soil–bentonite mix to stop preferential flow

Maintenance

Granular matrix sensors are maintenance free

FIGURE 6.5 Granular matrix sensor Dimensions: about 70 mm long and

20 mm diameter.

Granular matrix sensor

GBLite, Watermark

Figure 6.5

Potential limitations

> Manually read A logging system is available

> Measures soil water tension, which is a good indication of when to irrigate,

not how much.

> Does not work well in sandy soils, where the moisture drains more quicklythan the sensor can equilibrate

> If the soil becomes too dry you must remove and rewet the sensor

Positive points

> Simple and cheap

> Can read to a wide range of soil water tensions (10 to 200 kPa) therefore goodfor a range of soils and irrigation management strategies

> Measures soil water tension – more meaningful from a plant-stress aspect

> Buffers against salinity effects

SOIL MATRIC POTENTIAL THERMAL HEAT SENSOR Campbell Scientific 229

Methodology

The 229 is an example of a volumetric SWC measuring device embedded in a der of porous ceramic material, resulting in a composite instrument that measures soilwater tension (Figure 6.6)

cylin-The device uses the heat-pulse concept, which is explained further in the section

on ‘Heat dissipation’, to determine the water content of the ceramic material, which

in turn is in equilibrium with the water tension of the surrounding soil A heating ment is placed inside a hypodermic needle, and the ceramic material surrounds theneedle When a constant power is applied to the heater, the temperature increase inthe vicinity of the needle is related to the thermal conductivity of the material, which

ele-in turn is dependent on the amount of water present Practically, the device ture is measured before and after the heater is powered for 24 seconds The change intemperature is the only measurement required

tempera-The sensor can read from saturation to air-dry soil, but this capability is limited bythe extent of the calibration (typically –1500 kPa)

These are installed in an augered hole Surround the sensor with fine soil, andbackfill the hole with a soil–bentonite mix to stop preferential flow

FIGURE 6.6 Campbell Scientific’s 229 sensor.

Soil matric potential

thermal heat sensor

Campbell Scientific 229

Figure 6.6

The 229 is provided with a calibration that relates the measured change in ture to the soil water tension (kPa) This calibration is enough for tasks where meas-urement changes are more important than absolute values, but individual calibration

tempera-is recommended if you need greater accuracy

Data handling

The operation of the 229 must be controlled by a sophisticated data logger capable ofapplying a timed voltage and measuring thermocouple temperatures Such loggerscan also be programmed with a calibration equation to give the soil water tension as adirect output These data can then be downloaded to a PC spreadsheet

Maintenance

No maintenance is required

Possible limitations

> Requires a sophisticated data logger and a knowledge of logger programming

> Measures soil water tension, which is good indication of when to irrigate, not

> Measures a wide range of tensions (limited only by the calibration range)

> Data logger operation enables automatic data collection

> Not affected by salinity

EQUITENSIOMETER

Methodology

The equitensiometer (Figure 6.7) consists of a ThetaProbe®(see later in this section)embedded in a specially formulated porous material The ThetaProbe uses capaci-tance technology to measure the water content of the porous material, which equili-brates with the matric potential of the surrounding soil The measured volumetricSWC is then converted to matric potential via a predetermined water-retention rela-tionship

Bury the sensor at the required depth Make sure you have good soil contact: youmay need to surround the instrument with a small amount of fine soil Backfill thehole with bentonite to stop preferential flow

FIGURE 6.7 Equitensiometer (length = 200 mm, diameter = 40 mm).

Equitensiometer

Figure 6.7

Equitensiometers are delivered pre-calibrated You can check the calibration, but youmust return the unit to the manufacturer if it needs recalibrating The calibration isclaimed to be stable for two years The measurement range of the instrument is 0 to

1000 kPa Greater accuracy (± 5%) is achieved in conditions > 100 kPa Above thispressure an accuracy of ± 10 kPa is stated

Data handling

A handheld display (ThetaMeter®) is provided This applies the operating voltage(5–15 V DC) and outputs raw voltage Alternatively, a voltmeter and DC power sup-ply or standard logger can be used and calibration applied in a spreadsheet

> More suited to drier soil

Frequency domain reflectometry (capacitance)

Methodology

The EnviroSCAN system (Figure 6.8) consists of an array of capacitance sensorsinstalled at different depths within a PVC access tube The sensors are connected bycable to a central data logger, which powers the probes with a solar panel A range oftelemetry options can be used to offset the cable length All sensors in the same accesstube share one electronic measuring circuit, located at the top of each tube You canset reading intervals as close as 1 minute The standard probe lengths are 0.5, 1.0 and1.5 m, and the maximum number of tubes per data logger is eight Each data loggercan reference up to thirty two sensors in a 500 m radius

Installation technique is critical to the performance of devices that use the tance technique The manufacturers of EnviroSCAN have developed equipment andtechniques that are claimed to eliminate this problem Once installation has been com-pleted the equipment does not have to be disturbed again

Frequency domain

reflectometry

(capacitance)

EnviroSCAN

Data handling

All data are collected at the central data logger To view the data you must downloadthem to a PC using proprietary software You either take a laptop to the logger, takethe removable logger to the PC, or use telemetry to transfer the data straight to the

PC The software provides several presentation options, including time-series profile water content and separate sensor readouts Irrigation target setpoints may befed in for field capacity and lower-limit water contents

total-Maintenance

The equipment contains sensitive circuitry, so the most important maintenance task

is to stop moisture getting into the access tubes The sealing caps have gaskets Silicagel bags inside the tubes must be changed regularly

You should get the local distributor to arrange an annual maintenance check Thismight include battery charging, gasket changing and backing-up of data

Potential limitations

> Training and support are required Skill is needed to interpret the results

> Computer and software are required

> Not portable Sensors are fixed into access tubes

> If used with annual crops, you may need to remove the cabling and tubeswhen the crop is finished

> Measurement is very sensitive to the technique of access-tube installation

FIGURE 6.8 Sentek EnviroSCAN probe, showing measurement fields (left) and logger (right).

Figure 6.8

A user manual describes all operating features, including installation procedures.

Calibration

Diviner uses a similar universal calibration to the EnviroSCAN Customised tion can also be input for each depth increment at each site after you have performedthe same soil-sampling operation as for the EnviroSCAN Claimed accuracy is ±0.5%

calibra-Data handling

A time-series record of up to 99 sites can be stored in the handheld logger The datacan be presented either graphically or numerically Irrigation setpoints can be input

to indicate the range of soil moisture allowable to avoid crop stress

Although not required, a PC can be used to download, store and view data in dard spreadsheets

stan-Maintenance

None necessary The handheld logger is powered by a rechargeable battery

Potential limitations

> Portable manual recorder Logging not possible

> Some skill required to interpret results

> Measurement is very sensitive to the technique of access-tube installation

> The effect of salinity is unclear

Positive attributes

> Non-radioactive, unlike the neutron probe

> Economical way of covering many sites

> Rapid, easy measurement

> Avoids expensive, sensitive instruments being left in the field

FIGURE 6.9 Sentek Diviner 2000 sensor and display unit.

Diviner 2000

Figure 6.9

C-PROBE ®

Methodology

The C-Probe (Figure 6.10) is based on a very similar system to EnviroSCAN It has anarray of capacitance sensors installed at different depths within a PVC access tube The C-Probe has been designed to use the Adcon®UHF radio telemetry system TheAdcon system is not sensor specific and can carry data from a variety of instruments.Software has been designed to collect weather, soil moisture and other data to provide

an integrated system for both irrigation scheduling and disease prediction

The C-Probe is available in 0.5, 1.0, 1.5 m and longer lengths There is a maximumnumber of six sensors per tube/telemetry unit

Installation issues and procedures are similar to those for the EnviroSCAN Theonly cabling needed is from the sensor to the radio unit Both the measurement andtelemetry systems are powered by a small solar cell

Calibration

C-Probe can use a universal calibration equation similar to that used by theEnviroSCAN Alternatively, users can select calibrations for sand, loam, clay andother soil types or provide their own calibration While a universal calibration equa-tion is enough for many irrigation scheduling applications, the flexibility of beingable to fine tune the calibration for each sensor depth can be valuable in situationssuch as duplex soils If you need absolute volumetric data, do a specific calibration

An information kit and software module are provided for users who need a higherdegree of calibration accuracy

Data handling

The data-collection frequency is set by the user Each telemetry unit automaticallytransmits data to a central receiver capable of handling 95 field units The data areautomatically available to the user, either by having a desktop computer connected tothe receiver or via automatic download through a modem connection from other desk-tops or laptops The C-Probe and Adcon software provide several presentationoptions, including time-series graphs and statistics showing total profile water contentand separate sensor readouts Irrigation target setpoints can be input for field capaci-

ty and lower-limit water contents, as can a comprehensive set of agronomic markersand crop-stage markers

FIGURE 6.10 C-Probe with Adcon telemetry unit.

C-Probe

Figure 6.10

The equipment contains sensitive circuitry, so the most important part of nance is to stop moisture getting into the access tubes The sealing caps have O rings,and silica gel bags inside the tubes need to be checked at least twice a year andreplaced if necessary

mainte-Get your local distributor to do a maintenance check every two years This willinclude checking the O ring, swapping the silica gel bag, checking the battery andsolar panel performance and checking all connections

Potential limitations

> Training and support are required Basic skill is needed to interpret results

> Computer and software are required

> If used with annual crops, you may need to remove aboveground equipmentwhen the crop is finished

> Measurement is very sensitive to the technique of access-tube installation

Positive points

> Robust, repeatable measurements

> The only cabling required is from the sensor to the telemetry unit

> Precise depth resolution due to disc-like zone of influence

> Automatic operation reduces labour

> Continuous recording

> Infiltration rate, root activity and crop water use are easily interpreted

> Can monitor multiple depths at the same time

> Well suited to permanent plantings

The LCD display performs all functions of displaying and storing information, aswell as calibration The display can be linked to a PC for data storage and viewing,although this is not essential

Data handling

The display can store data for up to 48 profiles (times) from 54 sites, with sixteendepths for each site Data can be displayed as either a volumetric SWC value or a his-tograph The histographs are used to estimate the time interval before the next irri-gation cycle, and also to display the usage pattern from different depths in the soil bythe plants being irrigated You can also view summed graphs that indicate total avail-able water in the indicated profile depth

Gopher

The Gopher soil moisture profiler and soil moisture sensor are not waterproof: neverhandle them with wet hands or leave them exposed to the weather or irrigation sprin-klers Maintenance is centred on ensuring the access tubes are moisture free The equipment is fragile: handle it with care Never use the sensor staff cable topull the staff Always unplug the nine-way connector by holding the body of the plug.Never leave the Gopher or the sensor unprotected in full sunlight This will cause

an excessive temperature rise and may damage the LCD display in the Gopher.Excessive temperature increases in the sensor can produce unstable readings because

of expansion of the PVC housing

Potential limitations

> Portable manual recorder Logging is not possible

> Some skill required to interpret results

> Measurement is very sensitive to method of access tube installation

> The effect of salinity is unclear

> The equipment is not waterproof

FIGURE 6.11 The Gopher manually operated soil-moisture monitor.

Figure 6.11

BUDDY ®

(by Robert Hoogers, NSW Agriculture, Yanco)

Methodology

The Buddy consists of a series of Gopher capacitance sensors permanently placed in

an access tube for automatic time-series logging of profile SWC (Figure 6.12) Thesensor-connecting rods are manufactured to provide sensor-spacing distances of 10,

20, 30 and 40 cm This allows the sensor string spacing to be set up with multiples of

10 cm intervals The rods include a connector that provides the electrical connectionfrom the Buddy data recorder through to each sensor

The standard configuration includes four sensors and is expandable to eight

Calibration

Two methods for calibration are available The first uses soil sampling to gain a metric SWC The second is to use a Gopher to calibrate the profile at ‘Field Capacity’.The resulting coefficients are then fed into the Buddy software

volu-Data handling

The Buddy logger must be downloaded to a PC, either via a laptop or by taking the ger to the PC Two graph types are available: summed profile and separate multi-line

log-Maintenance

Always keep the plug and thread on the connecting rod very clean Foreign material

on the thread or connector plug such as soil will cause permanent damage and will not

be covered by the equipment warranty

Check the tube regularly to make sure there is no moisture in it

Potential limitations

> Training and support are required Skill is needed to interpret the results

> Computer and software are needed

> Not portable Sensors are fixed into access tubes

> If used with annual crops you may need to remove the sensors when the crop

is finished

> Measurement is very sensitive to access-tube installation technique

> The effect of salinity is unclear

FIGURE 6.12 Buddy in situ soil moisture monitor, showing logger and sensors.

Buddy

Figure 6.12

Positive points

> Robust, repeatable measurements

> Precise depth resolution due to disc-like zone of influence

> Automatic operation reduces labour

> Continuous recording

> Can monitor multiple depths at the same time

> Well suited to permanent plantings

Methodology

The Aquaterr consists of a single capacitance probe on the end of a rod about 1 m long.The rod is pushed into the soil to the desired depth, and the digital readout on top ofthe instrument gives the volumetric SWC at that depth

Calibration

The Aquaterr does not give absolute volumetric SWC The SWC value is a 0–100 ing, which is then related to a scale describing five moisture conditions from saturat-

read-ed to permanent wilting for three soil types (clay, loam and sand) The analog version

of the scale is shown in Figure 6.13 Aquaterr probes that measure salinity and perature are also available

> Penetrating dry soils is difficult

> Hand-recording of data is time consuming, and a constant measurement tion is difficult to maintain for regular reading

posi-> Interpretation of results may be confusing as this requires a good knowledge

of soil textual changes within the measurement areas

> The effect of salinity is unclear

THETAPROBE ®

Methodology

The ThetaProbe (Figure 6.14) avoids the limitations of an access tube by using steelspikes driven into the soil as the capacitance plates Whereas the permanentlyinstalled multi-sensor products have one circuit that analyses each sensor serially,each ThetaProbe has its own measurement electronics within the probe head Theinstrument can be either inserted into the soil surface to make one-off readings, or

buried for continuous in situ readings If you bury it, you should install it with an

extension tube (Figure 6.14) These tubes can be left in the ground and theThetaProbe inserted or removed when required

Calibration

The probe outputs a measurement in volts A calibration is then applied to the rawvoltage to give volumetric water content The literature states there is a virtually lin-ear relationship between the voltage (0 to 1 V) and the SWC (0 to 0.5 m3/m3) Two

‘generalised’ calibrations for ‘mineral’ and ‘organic’ soils are provided; these tee an accuracy of 5% (0.05 m3/m3) Use a two-point calibration if you want to achieve

guaran-an accuracy of 1% (0.01 m3/m3)

Data handling

A handheld display (ThetaMeter) is provided This applies the operating voltage (5 to

15 V DC) and outputs either raw voltage readings or volumetric water content usingthe two ‘generalised’ calibrations Alternatively, you can use a voltmeter and DCpower supply or standard logger and apply the calibration in a spreadsheet

Maintenance

The manufacturer states that no maintenance is required

FIGURE 6.14 ThetaProbe (Top length = 200 mm, diameter = 40 mm.) Bottom shows installation suggestion and ThetaMeter.

ThetaProbe

Figure 6.14

Potential limitations

> Manually read

> Replication of circuitry adds to the expense when using instrument arrays

> Hard to push probe into dry soil

> Minimal salinity effect

Positive points

> Inserting probes into soil greatly enhances contact

> Signal processing is completed at the instrument, leaving a simple voltageoutput

> The instrument arrays can be spatially distributed, for example, throughout aroot zone

NETAFIM SOIL MOISTURE DATA COLLECTOR

Methodology

The Netafim soil moisture data collector consists of up to three 3-rod probes

connect-ed to a data logger (Figure 6.15) The system uses a capacitance technique where theprobes are the plates and the surrounding soil is the dielectric

A switching circuit actuates the charging and discharging of the capacitor betweentwo constant limits The frequency of this cycle is inversely proportional to the watercontent of the soil This output is fed into a frequency-to-voltage converter that inte-grates the frequency into a proportional voltage, which may be read with a voltmeter.The probes are buried at the required depth

FIGURE 6.15 Netafim soil moisture data collector.

Netafim soil moisture

data collector

Figure 6.15

The sensor output data is presented in ‘soil moisture units’ with a practical range of

30 to 80 As with other sensors that do not rely on absolute soil-water-content urement, ‘calibration’ refers to identifying the irrigation full and refill points This isdone by analysing several dry-down cycles These show the point where post-irrigationdrainage ends (full point) and where crop water extraction from the profile slows(refill point)

> The probe is insensitive at high water contents

> It is hard to relate the output units to the irrigation amount applied or toevapotranspiration calculations

The salinity effects are not stated by the manufacturer (see first positive point below)

Positive points

> Unaffected by salinity in normal range of irrigation water/soil

> Simple, self-contained system

> Relatively cheap